The present invention relates generally to welding and particularly relates to welding on stress-crack sensitive materials in a manner enabling subsequent conventional fusion welding without the risk of helium-induced or other types of hot cracking in the fusion weld area.
There are various types of conventional fusion welding processes, such as electric arc, laser beam, or electron beam welding. In those processes, a molten pool of hot metal is formed, either by melting a substrate or adding a filler metal, or both. Materials, however, are oftentimes sensitive to hot cracking. Hot cracking of the welded surface is typically caused by strains and stresses due to contraction on cooling, i.e., during the phase change from liquid hot metal to a solid state. An extreme but actual example of hot cracking sensitive materials is fusion welding on material containing higher levels of helium, such as in permanent portions of older nuclear reactor vessel internals near the fuel core. In neutron irradiated austenitic stainless steels with significant boron content (which is susceptible to transmutation to helium), the helium in the weld materials causes several adverse effects through changes in mechanical properties. For example, when high helium content materials are exposed to the heat of a welding cycle, the high temperature allows the helium to diffuse rapidly to grain boundaries which form voids which, in turn, weaken the material resulting in hot cracking. Even for known low heat input fusion welding processes, the capability to reliably weld without hot cracking is limited to materials having relatively low helium levels. Hot cracking is also not limited to materials having a helium content but constitutes only one type of material in which hot cracking occurs. The hot cracking problem is also compounded by the typically high tensile temporal and residual surface stresses caused by the fusion processes. This adverse stress situation in the as-welded condition is characteristic of all conventional fusion welding processes and applications, especially for the heavy section thicknesses of materials generally found in permanent nuclear vessel internals and for the vessel wall itself or its attachments. It is effectively impossible to provide sufficiently low heat in the fusion welding process to avoid hot cracking, while still having a viable fusion welding process.
In addition to the hot cracking problem during cooling of the weld pool, stress corrosion cracking (SCC) can occur in materials susceptible to thermal or neutron sensitization when used in aggressive environments such as oxygen or halogen containing high temperature nuclear reactor water or moderator. This type of environmentally induced cracking occurs when the level of surface residual stress becomes sufficiently tensile as is typically the case for conventional fusion welding practice.
In accordance with a preferred embodiment of the present invention, there is provided a low-cost reliable method for making full strength structural welds on materials which are susceptible to hot cracking enabling subsequent conventionally applied fusion welding processes. For example, highly neutron irradiated substrate materials as in a nuclear reactor vessel can be subsequently fusion welded without risk of helium-induced or other types of hot cracking in the fusion weld area. Particularly, a non-fusion electrical resistance or solid state weld process with very low local heat input is employed in conjunction with application of localized forces to the weld compressively stressing the weld in a crack-susceptible or sensitive substrate during both the welding and cooling stages. Additionally, by using a highly conductive electrode as a heat sinking source, the present process simultaneously affords residual compressive stresses in the cladding or substrate to mitigate subsequent environmentally induced failures such as stress corrosion cracking. The cladding may be a separate sheet of material overlying the substrate or can be formed by fusing the surface of the substrate where the cladding is a sheet metal overlay. The electrical resistance welding can be performed about the margins of the overlay or throughout portions or all of the sheet metal overlay and substrate, thus protecting the underlying substrate from exposure to the external environment. For example, the sheet metal overlay may seal the underlying substrate from an aggressive water environment in a nuclear reactor vessel minimizing or eliminating problems associated with stress corrosion cracking of the underlying substrate.
It will be appreciated that portions of the weld which attach to the substrate are the most susceptible to hot cracking, particularly in materials containing substantial quantities of helium, in the event fusion welds are performed in that region. By employing electrical resistance heating at low heat input to the extent that a substantial portion or all of the materials joined remain below the liquidus temperature, the heat does not penetrate sufficiently for the helium to defuse and affect grain boundaries which cause hot cracking. That is, the electrical resistance weld of the preferred embodiment hereof provides insufficient heat in a temperature regime which can lead to hot cracking as a result of tensile stresses occurring during and after welding. Additionally, compressive forces are applied during the welding process at low temperature to provide triaxial compressive stresses in the weld which are gradually relieved and prevent thermal shrinkage due to induced tensile stresses (hot cracking).
Particularly, in the preferred embodiment of the present invention an electrode is applied to a weld surface, e.g. a thin sheet material overlying a substrate and heat and compression forces are applied to the weld. Because of the relatively small size of the plastic weld nugget during application of heat from the electrode, the compressive surface stress at the electrode contact area leads to triaxial compressive stresses in the contained portions of the weld nugget. The electrode also serves as a heat sink for the weld such that welding is performed for a predetermined time and over a temperature and force regime to substantially preclude hot cracking in the weld.
In a preferred embodiment according to the present invention, there is provided a method of welding on a substrate susceptible to hot cracking comprising the steps of (a) applying a non-fusion electrical resistance weld to the substrate susceptible to hot cracking, (b) while performing step (a), applying a localized force to compressively stress the weld and (c)performing steps (a) and (b) for a predetermined time and over a temperature and force regime to substantively preclude subsequent hot cracking in the weld.
In a further preferred embodiment according to the present invention, there is provided a method of welding on a substrate susceptible to hot cracking comprising the steps of (a) applying a low heat input solid state weld to the substrate susceptible to hot cracking, (b) while performing step (a), applying a localized force to the weld compressively stressing the weld and (c) performing steps (a) and (c) for a predetermined time and over a temperature and force regime to substantially preclude subsequent hot cracking in the weld.